From e74c74f9e51deb725e72d129084ba8252d47222d Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Toke=20H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com> Date: Thu, 24 Jun 2021 18:05:53 +0200 Subject: [PATCH] doc: Give XDP as example of non-obvious RCU reader/updater pairing MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit This commit gives an example of non-obvious RCU reader/updater pairing in the guise of the XDP feature in networking, which calls BPF programs from network-driver NAPI (softirq) context. Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20210624160609.292325-4-toke@redhat.com --- Documentation/RCU/checklist.rst | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst index 07f6cb8f674d4..01cc21f17f7bd 100644 --- a/Documentation/RCU/checklist.rst +++ b/Documentation/RCU/checklist.rst @@ -236,8 +236,15 @@ over a rather long period of time, but improvements are always welcome! Mixing things up will result in confusion and broken kernels, and has even resulted in an exploitable security issue. Therefore, - when using non-obvious pairs of primitives, commenting is of - course a must. + when using non-obvious pairs of primitives, commenting is + of course a must. One example of non-obvious pairing is + the XDP feature in networking, which calls BPF programs from + network-driver NAPI (softirq) context. BPF relies heavily on RCU + protection for its data structures, but because the BPF program + invocation happens entirely within a single local_bh_disable() + section in a NAPI poll cycle, this usage is safe. The reason + that this usage is safe is that readers can use anything that + disables BH when updaters use call_rcu() or synchronize_rcu(). 8. Although synchronize_rcu() is slower than is call_rcu(), it usually results in simpler code. So, unless update performance is -- GitLab